Martin Sheen plays fictional Democratic President Josiah Bartlet, now serving his second term on the NBC political drama, which returns for its sixth season on Oct. 20.

Alda hasn't signed on to a TV series commitment since his days as the renegade medic Capt. Benjamin "Hawkeye" Pierce on CBS' "M.A.S.H," which ran from 1972-1983.

Howard said Monday that Alda decided to guest star on "The West Wing" because of his experience working with producer John Wells on "ER." Wells is the producer of both shows.

Alda was nominated for an Emmy in 2000 for his guest spot as the Alzheimer's-afflicted Dr. Gabriel Lawrence on the NBC medical drama.

Alda, 68, plays a senator in the upcoming Howard Hughes biopic, "The Aviator," directed by Martin Scorsese. His films also include "California Suite," "Manhattan Murder Mystery" and "Flirting With Disaster."